The routing process of a network is not a simple task. It is convenient to think of a network as layers on top of one another, each layer representing a specific task area of the network (FIG. 1). Each layer comprises nodes and transmission links which are needed to represent the function of the layer. Often transmission links are called edges as in this text as well. Usually the top layer (1) describes logical connections of the network, how a single node (for example a base station) sees the network, i.e. transparent connections. Correspondingly, the bottom layer describes the physical network (2), where nodes and real transmission lines have been located. Between the top and the bottom layers there are several sublayers (3), each of them representing either the logical or physical network of a specific technology, such as 2 Mbit/s frame connections. The route for a connection in a layer under review must always be found in the layer below. So, the routes for the logical layer connections must be found in the layer below, and connections of this layer must have routes in the layer below this layer, and so on. Finally, all connections have routes in the bottom layer.
If a connection is desired to be protected, two separate routes must be found between endpoints of the connection. Sometimes it happens that the routing from the layer above to the layer below fails, i.e. both routes go through the same edge, as the situation is in FIG. 2. The left ring shows a situation where two connections between nodes A and B go through the same edges, when there is no spare route for either one of the connections. If protection is desired to be a real all-time protection, then both connections must carry the same signal. The right ring shows a situation where the connections go through separate edges all the way between nodes A and B. Usually, a protection switch is located in the endpoints (A and B in FIG. 2) where a protected route (two separate connections) start and end. The protection switch splits the signal of the connection into two signals. Taking into account the above-mentioned matters, protected connections must be routed through all layers in a way that there exist two separate physical routes between endpoints.
The objective of the invention is to make it possible to create protected connections through all layers, eliminating the above-mentioned drawback. This is achieved in a way described in the claims.